1. Field of the Invention
The invention relates to data communications and, in particular, to multitone modulation such as employed digital subscriber line (DSL) communications.
2. Description of the Related Art
A digital subscriber line (DSL) system uses the existing twisted-pair telephone wires to carry data between central offices (CO) and customer premise equipment (CPE). Data transmissions from CO to CPE (downstream direction) and from CPE to CO (upstream direction) usually occupy different bands (or sub-channels), although in some echo cancelled implementations, both upstream and downstream transmissions may occupy the same band. One of the modulation techniques used in a DSL system is called discrete multitone modulation (DMT). DMT techniques partition the transmission channel in each direction into a bank of orthogonal, memoryless subchannels, and transmit data through each sub-channel independently.
In general, a DMT-based DSL connection is established by training receivers, probing channels, and allocating varying number of bits to sub-channels for data transmission. Initially, signals are sent in both directions to train the receivers and to probe the channels. Then, a signal-to-interference-plus-noise ratio (SINR) is calculated on each sub-channel, and bits are allocated to each sub-channel based on its SINR. In this context, interference may include echoes, crosstalk, radio frequency interference (RFI), etc. To accommodate a wide range of loops and interference, training signals usually include a range of tones (possibly all the available tones) within the respective bands. The tones that are actually used for data transmission are determined based on channel probing results and bit-loading algorithms.
Training signals have typically been selected independent of loop impairments. Accordingly, there are often tones in the training signals that are not used in the final data transmission. Potentially, many tones are included in the training signals but not employed in data transmissions over a given sub-channel. These extra tones in the training signals do not contribute to actual data transmission performance. However, they may degrade performance in the receiving direction by creating unnecessarily high echoes. In the case of long subscriber loops, communication equipment may be particularly sensitive to such degradation. In addition, they may degrade performance in the other wire pairs by creating unnecessarily high crosstalks into those wire pairs.